Method for heat treatment of silicon wafers and silicon wafer

ABSTRACT

According to the present invention, there are provided a method for heat treatment of silicon wafers wherein a silicon wafer is subjected to a heat treatment at a temperature of from 1000° C. to the melting point of silicon in an inert gas atmosphere, and temperature decreasing in the heat treatment is performed in an atmosphere containing 1-60% by volume of hydrogen, a method for heat treatment of silicon wafers under a reducing atmosphere containing hydrogen by using a rapid heating and rapid cooling apparatus, wherein temperature decreasing rate from the maximum temperature in the heat treatment to 700° C. is controlled to be 20° C./sec or less, and a silicon wafer which has a crystal defect density of 1.0×10 4  defects/cm 3  or more in a wafer bulk portion, a crystal defect density of 1.0×10 4  defects/cm 3  or less in a wafer surface layer of a depth of 0.5 μm from the surface, a crystal defect density of 0.15 defects/cm 2  or less on a wafer surface and surface roughness of 1.0 nm or less in terms of the P-V value. By these, crystal defects in wafer surface layers can be reduced by a simple method with a small amount of hydrogen used without degrading microroughness of wafers.

TECHNICAL FIELD

[0001] The present invention relates to a method for heat treatment ofsilicon wafers, in particular, a method for heat treatment of siliconwafers that shows superior safety and can give silicon wafers of highquality.

BACKGROUND ART

[0002] As wafers for manufacturing semiconductor devices such assemiconductor integrated circuits, silicon wafers are mainly used. Insuch production of semiconductor devices, crystal defects which exist inwafer surface layers such as COPs (Crystal Originated Particles) can bementioned as one of factors that degrade the yield. If such crystaldefects exist in wafer surface layers, they may be a cause ofdegradation of electric characteristics of wafers. For example, intransistors of MOS structure, when high voltage is applied to a thermaloxide film formed on a wafer surface such as a gate oxide film, they maycause generation of dielectric breakdown of the oxide film.

[0003] As a further factor that worsens the yield of semiconductordevice production, microroughness on wafer surfaces can be mentioned. Itis known that microroughness that exists on wafer surfaces adverselyaffect carrier mobility directly under the gate oxide films (see ShinyaYamakawa, Hirai Ueno, Kenji Taniguchi, Chihiro Hamaguchi, KazuoMiyatsuji, Umbert Ravaioli, J. Appl. Phys., 79, 911, 1995). Insemiconductor devices, if degree of integration is increased, carriermobility must correspondingly be increased. Moreover, with recent use ofincreasingly higher driving frequency of CPU, write time and read timeof memories are required to be made higher. Therefore, it has come to beconsidered more important to make microroughness small in order toimprove carrier mobility.

[0004] As a method for reducing crystal defects of silicon wafer surfacelayers, elimination of the defects by annealing heat treatment and soforth have been performed. A typical example thereof is high temperaturehydrogen annealing. This method is a method for eliminating crystaldefects by performing annealing heat treatment in a hydrogen atmosphereat a high temperature (see Japanese Patent Laid-open Publication (Kokai)No. 6-349839).

[0005] However, although a heat treatment in a hydrogen atmosphere canreduce crystal defects in wafer surface layers, it has a drawback thatsurfaces of wafers will be etched by the heat treatment. For example,when a heat treatment is performed at 1200° C. for 60 minutes, about 0.5μm of silicon of wafer surface layers will be etched. For this reason,thickness of the portions of wafer surfaces with few crystal defects(defect-free layer) becomes small.

[0006] Furthermore, it is very dangerous to handle hydrogen gas at ahigh concentration at such a high temperature over a long period oftime. Thus, it cannot be practically used without solving the problem ofsafety.

[0007] Therefore, there has also been proposed a method for eliminatingcrystal defects of wafer surface layers by performing a heat treatmentusing an inert gas such as argon for the atmosphere. However, althoughthe crystal defects in wafer surface layers can be reduced by thismethod without etching wafer surface layers, it has a drawback that itworsens microroughness on wafer surfaces compared with that before theheat treatment.

[0008] In addition, there is also an abuse that local etching comes tobe likely to occur due to the influence of a small amount of oxygen inthe atmosphere, and thus haze is generated.

[0009] The term “haze” used herein is an index of surface roughness, andmeans periodical waviness having a period of several to several tens ofnanometers on the wafer surfaces. It is surface roughness that can besemi-quantitatively evaluated as a haze level of a whole wafer surfaceby scanning the whole wafer surface with a particle counter mainlyutilizing a laser, and measuring strength of scattered reflectionthereof.

[0010] Another method for avoiding the danger of hydrogen gas, there hasalso been contemplated a method utilizing a heat treatment with ahydrogen atmosphere and a heat treatment with an inert gas atmospheresuch as argon in combination. This method comprises first performing aheat treatment of wafers in an inert gas atmosphere, and then performinga heat treatment of the wafers in a hydrogen atmosphere (see JapanesePatent Laid-open Publication No. 4-167433).

[0011] However, the heat treatment in an atmosphere containing hydrogenat the same temperature as the preceding heat treatment in an inert gasatmosphere will eventually etch wafer surfaces, and thus the defect-freelayer thickness will become small.

[0012] Further, Japanese Patent Laid-open Publication No. 7-235507discloses a method which comprises performing a heat treatment in aninert atmosphere, wherein hydrogen is introduced into the atmosphereduring the temperature increasing or temperature decreasing period ofthe heat treatment. However, this method was accomplished with thepurpose of preventing generation of slip dislocations in wafers byintroducing hydrogen having heat conductivity higher than that of inertgas during the temperature increasing or temperature decreasing, andthis method is not for eliminating crystal defects which exist in wafersurface layers or improving microroughness on wafer surfaces.

[0013] That is, this method simply comprises continuously introducing 1liter/minute of hydrogen during the temperature increasing anddecreasing, and an optimum composition of the heat treatment atmosphereduring the temperature increasing and decreasing is unknown. Therefore,even if this method is used, the etching amount of the wafer surfacesmay become large, or microroughness may be worsened. Thus, crystaldefect density and surface roughness cannot be improved simultaneously.

[0014] As described above, among the conventional heat treatmentmethods, there are no method for reducing crystal defects of wafersurface layers without etching wafer surface layers and withoutdegrading microroughness of wafers, with a little amount of hydrogenused. Therefore, it is desired to develop an effective method.

[0015] Furthermore, in the hydrogen annealing, the heat treatment isusually performed under a hydrogen gas atmosphere by increasingtemperature at a temperature increasing rate of 1-10° C./min,maintaining a temperature of from 950° C. to the melting point ofsilicon for several hours, and then decreasing the temperature at atemperature decreasing rate of 2-5° C./min (for example, Japanese PatentPublication (Kokoku) No. 5-18254 and Japanese Patent Laid-openPublication No. 6-295912). However, this heat treatment method has adrawback that the heat treatment requires a long period of time.

[0016] Therefore, it has been proposed a method for heat treatment usingan apparatus for rapid heating and rapid cooling (Rapid ThermalAnnealer, also abbreviated as “RTA apparatus” hereinafter) in order toshorten the heat treatment time etc. For example, in Japanese PatentApplication No. 10-82606, the inventors of the present inventionpreviously proposed a method for heat treatment of silicon wafers undera reducing atmosphere using an RTA apparatus, and proposed a method forheat treatment that can, in particular, reduce the COP density onsurfaces of silicon wafers.

[0017] This method comprises a heat treatment of silicon wafers within atemperature range of from 1200° C. to the melting point of silicon for1-60 seconds under a reducing atmosphere. In this method, it is furtherpreferred that 100% hydrogen or a mixed atmosphere of hydrogen and argonis used as the reducing atmosphere, and the heat treatment time isselected to be 1-30 seconds.

[0018] Further, it was found that COP density on surfaces of siliconwafers was markedly reduced and one of the electric characteristics,oxide dielectric breakdown voltage (Time Zero Dielectric Breakdown:TDDB), was also markedly improved by this method.

[0019] However, this method has a drawback that the aforementionedsurface roughness on wafer surfaces after the heat treatment, calledhaze, may be degraded.

[0020] In addition, as mentioned above, it is known that surfaceroughness on wafer surfaces such as haze closely relates to performanceand reliability of devices as a factor which greatly affects theelectrical characteristics such as oxide dielectric breakdown voltageand carrier mobility (see Shinya Yamakawa et. al., J. Appl. Phys. 79,911, 1996), and haze on wafer surfaces is considered a big problem.

[0021] Therefore, in Japanese Patent Application No. 10-176693, theinventors of the present invention proposed a method wherein the RTAheat treatment is performed with two or more divided steps as a methodfor solving that problem. In this method, a heat treatment of apreceding step is performed with the purpose of reduction of COPs, and aheat treatment of a subsequent step is performed in order to improvesurface roughness on wafer surfaces such as haze.

[0022] Since this method can sufficiently improve haze while reducingCOPs, it is a very useful method. However, because it requiresperforming two or more steps of high temperature heat treatments in anRTA apparatus, it has a drawback of complicated process steps. Suchcomplicated process steps lead to increase of cost due to decrease ofthroughput. Therefore, it is desired to be further improved.

DISCLOSURE OF THE INVENTION

[0023] The present invention was accomplished in view of such problemsas mentioned above, and an object of the present invention is to providea method for heat treatment that can reduce crystal defects in wafersurface layers without etching wafer surface layers and withoutdegrading microroughness on wafer surfaces with a small amount ofhydrogen used.

[0024] Another object of the present invention is to provide a methodfor heat treatment of silicon wafers using a rapid heating and rapidcooling apparatus, which can reduce COPs and haze of wafer surfaces in asimpler manner.

[0025] In order to achieve the aforementioned objects, the presentinvention provides a method for heat treatment of silicon wafers,wherein a silicon wafer is subjected to a heat treatment at atemperature of from 1000° C. to the melting point of silicon in an inertgas atmosphere, and temperature decreasing in the heat treatment isperformed in an atmosphere containing 1-60% by volume of hydrogen.

[0026] Thus, by subjecting a silicon wafer to a heat treatment attemperature of from 1000° C. to the melting point of silicon in an inertgas atmosphere, crystal defects of the wafer surface layer can beeliminated first. Then, by decreasing the temperature in an atmospherecontaining 1-60% by volume of hydrogen during temperature decrease ofthe heat treatment, microroughness can be improved thanks to migrationof silicon atoms on the wafer surface. In this method, since the amountof hydrogen gas used may be small, the safety of the heat treatment stepcan also be improved.

[0027] In the above method, the aforementioned inert gas atmospherepreferably consists of an argon atmosphere or an argon atmospherecontaining 30% by volume or less of hydrogen.

[0028] This is because argon is easily handled, and even when itcontains hydrogen, etching due to hydrogen contained in the atmospherehardly occurs if its concentration is 30% by volume or less, and theeffect of improving microroughness on the wafer surfaces will becomehigher to the contrary.

[0029] The present invention also provides a method for heat treatmentof silicon wafers under a reducing atmosphere containing hydrogen byusing a rapid heating and rapid cooling apparatus, wherein temperaturedecreasing rate from the maximum temperature in the heat treatment to700° C. is controlled to be 20° C./sec or less.

[0030] By employing such a simple method as described above, i.e., onlyby using a temperature decreasing rate of 20° C./sec or less from themaximum temperature in the heat treatment to 700° C., in a method forheat treatment of silicon wafers using a rapid heating and rapid coolingapparatus, haze can be improved while COPs of the wafer surface aresimultaneously reduced.

[0031] In the above method, it is preferred that the temperaturedecreasing rate in a region below 700° C. in the heat treatment shouldbe faster than the temperature decreasing rate from the maximumtemperature to 700° C.

[0032] By using a temperature decreasing rate in a region below 700° C.in the heat treatment faster than the temperature decreasing rate in aregion of from the maximum temperature to 700° C. as described above,the whole heat treatment time can be shortened, and thus the efficiencyof the heat treatment can further be improved.

[0033] In the above method, it is preferred that the aforementionedreducing atmosphere containing hydrogen should be 100% hydrogen or amixed gas atmosphere of hydrogen with argon and/or nitrogen.

[0034] Such a heat treatment atmosphere can surely reduce COP density onthe wafer surface and improve haze.

[0035] Moreover, in a silicon wafer subjected to the heat treatment bythe aforementioned method of the present invention, COP density on thewafer surface is decreased and haze is made small by the simple method,and thus the electric characteristics of the silicon wafer such as oxidedielectric breakdown voltage and carrier mobility can be improved.Therefore, extremely useful silicon wafers of extremely high quality canbe obtained with high productivity.

[0036] Specifically, for example, the silicon wafer can be a siliconwafer having a crystal defect density of 1.0×10⁴ defects/cm³ or more inthe wafer bulk portion, a crystal defect density of 1.0×10⁴ defects/cm³or less in the wafer surface layer of a depth of 0.5 μm from thesurface, a crystal defect density of 0.15 defects/cm² or less on thewafer surface and surface roughness of 1.0 nm or less in terms of theP-V value.

[0037] The term “wafer bulk portion” used herein means a portion ofwafer present at a depth exceeding 0.5 μm from the wafer surface.

[0038] Thus, the silicon wafer of present invention can be a siliconwafer with a low crystal defect density of 1.0×10⁴ defects/cm³ or lessin the wafer surface layer of a depth of 0.5 μm from the surface andless surface roughness of 1.0 nm or less in terms of the P-V value, eventhough it had a high crystal defect density during the growth of asilicon single crystal. In addition, since it contains crystal defectsrequired for gettering of impurities such as heavy metals in the bulkportion, semiconductor devices showing superior oxide dielectricbreakdown voltage characteristics and carrier mobility can be producedand the yield of the device production can be improved, if the devicesare produced by using the silicon wafer of the present invention.

[0039] As explained above, according to the present invention, there canbe obtained a silicon wafer having few crystal defects on the wafersurface, a large thickness of the defect-free wafer surface layer andless microroughness on the wafer surface by using a specifically definedoptimum composition of the heat treatment atmosphere in a method of heattreatment of silicon wafers. Therefore, the yield of the deviceproduction can be improved. In addition, since it becomes possible tominimize the amount of hydrogen used by the method of the presentinvention, the safety of the heat treatment operation can be secured.

[0040] In addition, according to the present invention, a heat treatmenthaving both effects for eliminating defects such as COPs on wafersurfaces and improving haze can be performed in an extremely simplemanner by using an improved temperature decreasing rate of the heattreatment in a method for heat treatment of silicon wafers using a rapidheating and rapid cooling apparatus. Thus, it becomes possible to obtainsilicon wafers of higher quality compared with conventional ones withlower cost in a simple manner.

BRIEF EXPLANATION OF THE DRAWINGS

[0041]FIG. 1 is a graph representing the relationship betweenatmospheric conditions during temperature increasing and temperaturedecreasing in heat treatment and P-V values of wafers after the heattreatment.

[0042]FIG. 2 is a graph representing the relationship between hydrogenconcentrations during temperature decreasing in heat treatment and P-Vvalues of wafers after the heat treatment.

[0043]FIG. 3 is a graph representing the relationship between hydrogenconcentrations during temperature decreasing in heat treatment and hazeof wafers after the heat treatment.

[0044]FIG. 4 is a graph representing the relationship between hydrogenconcentrations during temperature decreasing in heat treatment andetched amounts of wafers after the heat treatment.

[0045]FIG. 5 is a graph representing the relationship betweencompositions of atmosphere during retention at a constant temperatureand etched amounts of wafers after the heat treatment.

[0046]FIG. 6 is a graph representing the relationship betweentemperature decreasing rates and haze in heat treatment by an RTAapparatus.

[0047]FIG. 7 is a graph representing the relationship betweentemperatures at which, temperature decreasing rate is changed to ahigher rate and haze on wafer surfaces.

[0048]FIG. 8 is a schematic view of an exemplary heat treatmentapparatus for performing the heat treatment according to the presentinvention.

[0049]FIG. 9 is a schematic cross-sectional view of an exemplaryapparatus that can rapidly heat and rapidly cool silicon wafers.

BEST MODE FOR CARRYING OUT THE INVENTION

[0050] Hereafter, the present invention will be explained in moredetail.

[0051] The present invention was accomplished as a result of the presentinventors' various quantitative researches about heat treatmentconditions of silicon wafers, in particular, about compositions of heattreatment atmosphere, in which they found the optimum conditions.

[0052] The inventors of the present invention performed firstexperiments and investigation about influence of atmospheric conditionsduring temperature increasing and temperature decreasing in the heattreatment on the surface conditions of wafers. First, heat treatmentexperiments were performed, in which a plurality of silicon wafers ofthe same specification were prepared, inserted into a heat treatmentfurnace, heated to 1200° C. at a temperature increasing rate of 10°C./min, maintained at 1200° C. for 60 minutes, and then cooled at atemperature decreasing rate of 3° C./min. During the temperatureincreasing and temperature decreasing, the atmosphere was changed foreach wafer to perform the heat treatments, and microroughness of thewafers after the heat treatments was evaluated by measuring P-V values(maximum difference between peaks and valleys). The measurement wasperformed for 2 μm squares by using AFM (atomic force microscope). Theresults of the measurement are shown in FIG. 1.

[0053] From the results of FIG. 1, it can be seen that the P-V valueobtained for the case where the heat treatment was performed under theatmosphere consisting only of argon as conventionally known was degradedby about 4 times compared with the value for the case where the heattreatment was performed under the atmosphere consisting only ofhydrogen. Moreover, it can also be seen that this tendency was notaffected at all when each atmosphere during the temperature increasingwas changed to the other gas.

[0054] On the other hand, the results differed when the atmosphereduring the temperature decreasing was changed and it could be seen asfollows. That is, when the atmosphere during the temperature decreasingwas changed from the hydrogen atmosphere to the argon atmosphere, theP-V value was degraded in spite of the fact that the heat treatment wasperformed in the hydrogen atmosphere. In contrast, when the atmosphereduring the temperature decreasing was changed from the argon atmosphereto the hydrogen atmosphere, there was obtained a P-V value comparable tothat obtained by the heat treatment under the hydrogen atmospherealthough the heat treatment was performed under the argon atmosphere.

[0055] From these results, it was found that microroughness of waferswas not affected by the atmosphere during the temperature increasing atall, but the atmospheric conditions during the temperature decreasingdetermined microroughness of wafers. That is, even though the heattreatment is performed in the argon atmosphere, wafer surfacescomparable to those obtained from the heat treatment under the hydrogenatmosphere can be obtained by changing the atmosphere into the hydrogenatmosphere during the temperature decreasing.

[0056] However, if the hydrogen concentration in the atmosphere duringthe temperature decreasing is made too high, etching amounts of wafersare increased and it is not preferred. Therefore, the inventors of thepresent invention further performed experiments and examination aboutatmospheric conditions of the heat treatment. Experiments were performedin the same manner as in the aforementioned experiments, in which aplurality of silicon wafers of the same specification were prepared,heated at a temperature increasing rate of 10° C./min, maintained at1200° C. for 60 minutes, and then cooled at a temperature decreasingrate of 3° C./min. The heat treatments were performed with introducinghydrogen into the argon atmosphere during the temperature decreasing ateach of different volume ratios varying in the range of 1-100% by volumefor each wafer. Microroughness of the wafers subjected to the heattreatment was evaluated by measuring P-V values of the wafers. Theresults are shown in FIG. 2.

[0057] From the results of FIG. 2, the effect of improving surfaceroughness of wafers can be obtained if the amount of mixed hydrogenduring the temperature decreasing is 1% by volume or more. From this, itis concluded that microroughness of wafers can be improved with theminimum amount of hydrogen used by performing the heat treatment in aninert atmosphere such as argon, and by introducing 1% by volume or moreof hydrogen during the temperature decreasing.

[0058] Conversely, it was found that, if the amount of mixed hydrogen is1% by volume or less, the effect of improving surface roughness ofwafers is markedly reduced, and hence it is important that the amount ofmixed hydrogen should be 1% by volume or more.

[0059] Further, the inventors of the present invention performed similarmeasurement for haze of wafers. The results are shown in FIG. 3. Fromthe results of FIG. 3, it can also be seen that the effect of improvinghaze of wafers is obtained if the amount of mixed hydrogen during thetemperature decreasing is 1% by volume or more, like microroughness.

[0060] In addition, the inventors of the present invention performedmeasurement also about the relationship between the hydrogenconcentration of the atmosphere during the temperature decreasing andthe amount of etching of wafers. Evaluation of the amount of etching wasperformed by using SOI (Silicon On Insulator) wafers having a thicknessof 1 μm, measuring SOI film thickness before and after the heattreatments, and calculating the difference of them. The results of themeasurement are shown in FIG. 4. The ordinate of FIG. 4 representsarbitrary units, which are defined based on the amount of etchingobtained with 100% of hydrogen concentration as a reference.

[0061] As shown in FIG. 4, when the hydrogen concentration of theatmosphere during the temperature decreasing was 30% by volume or less,etching of wafer surfaces hardly occured. However, it was found that, ifthe hydrogen concentration exceeded 30% by volume, etching began to begenerated, and when it exceeded 60% by volume, the amount of etching ofwafers increased sharply.

[0062] The above results are summarized as follows. That is, the surfaceconditions of the wafers after the heat treatment can be improved byintroducing hydrogen into the atmosphere during the temperaturedecreasing of the heat treatment, and the sufficient effects ofimproving microroughness and haze of wafers can be obtained by ahydrogen concentration of 1% by volume or more. Further, it was foundthat the amount of etching within an acceptable range could be obtainedby using a hydrogen concentration of 60% by volume or less.

[0063] Further, the inventors of the present invention performedexamination also about the atmospheric composition during the retentionof the heat treatment temperature in the heat treatment. Since it wasfound that surface roughness of wafers could be improved by introducinga suitable amount of hydrogen into the atmosphere during the temperaturedecreasing as described above, no problem will be caused even if argonis used as the heat treatment atmosphere during the retention ofconstant temperature. The inventors of the present invention performedexperiments and examination about relationship between the amount ofetching of wafers and the atmospheric composition during the retentionof heat treatment temperature.

[0064] Heat treatment experiments were performed, in which a pluralityof SOI wafers having an SOI film thickness of 1 μm were prepared,inserted into a heat treatment furnace, heated to 1200° C. at atemperature increasing rate of 10° C./min, maintained at 1200° C. for 60minutes, and then cooled at a temperature decreasing rate of 3° C./min.The heat treatments were performed by varying the amount of hydrogenmixed into the argon atmosphere for each wafer during the retention ofthe constant temperature, and the etched amounts of SOI wafers after theheat treatments were measured. The temperature increasing and decreasingwere performed in an argon atmosphere. The results of the measurementare shown in FIG. 5.

[0065] From the results of FIG. 5, it can be seen that, if the hydrogenconcentration in the heat treatment atmosphere during the retention ofthe constant temperature is 30% by volume or less, etching of surfacesof wafers hardly occurs even after the heat treatment is performed at1200° C. for 60 minutes. Therefore, surface roughness of wafers can alsobe improved during the retention of constant temperature by introducing30% by volume or less of hydrogen into the atmosphere during thisretention of constant temperature without generating etching. On theother hand, it was found that, if the amount of introduced hydrogenexceeded 30% by volume, the amount of etching of wafer surfaces becamelarge and thus it was preferred that it should not exceed that value.

[0066] Furthermore, inventors of the present invention studied throughvarious experiments about heat treatment conditions that could reducethe density of COPs present on silicon wafer surfaces to improve oxidedielectric breakdown voltage as well as can decrease haze to improvecarrier mobility in a simpler manner compared with conventional methodsin a heat treatment method using a rapid heating and rapid coolingapparatus. As a result, the inventors of the present invention foundthat COP density and haze can be simultaneously reduced or decreased byusing a relatively low temperature decreasing rate of 20° C./sec or lessfor temperature decrease from the maximum temperature to 700° C. in theheat treatment.

[0067] It was conventionally thought that the effect of modifying wafersurfaces by heat treatment under a reducing atmosphere containinghydrogen should be determined mainly by the maximum temperature in theheat treatment and retention time at the temperature irrespective of useor disuse of an RTA apparatus, and the temperature increasing rate tothe maximum temperature and the temperature decreasing rate from themaximum temperature were not taken so much into consideration.

[0068] In particular, when an RTA apparatus is used, the influence oftemperature increasing and decreasing rates on wafer surfaces were notconsidered at all, since the times required for temperature increase anddecrease are extremely shorter compared with usual heat treatmentfurnaces of resistance heating type, and temperature increasing anddecreasing rates of 30-60° C./sec were usually used.

[0069] Therefore, the inventors of the present invention performedexperiments of heat treatment of silicon wafers using the heat treatmentmethod disclosed in Japanese Patent Application No. 10-82606. As aresult, it was found that, even in a heat treatment using an RTAapparatus, surface conditions of wafers after the heat treatmentsdiffered, in particular, the haze level greatly differed, if thetemperature decreasing rate from the maximum temperature differed.

[0070] The inventors of the present invention performed the followingexperiments in order to determine suitable temperature decreasingconditions in heat treatment of silicon wafers using an RTA apparatus.

[0071] Silicon wafers obtained by the Czochralski method were subjectedto heat treatments at 1200° C. for 10 seconds under 100% hydrogenatmosphere by using an RTA apparatus while varying the temperaturedecreasing rate during temperature decreasing from the maximumtemperature of the heat treatments, and haze of wafer surfaces wasmeasured.

[0072] For the heat treatments, an RTA apparatus (rapid heating andrapid cooling apparatus Model SHS-2800, Steag Microtec International)was used.

[0073] The silicon wafers used were those obtained by slicing a siliconingot manufactured by the Czochralski method and subjecting the slicedwafers to mirror polishing in a conventional manner, and having adiameter of 6 inches and crystal orientation of <100>.

[0074] Haze was measured by using a particle counter, LS-6030 (tradenameof Hitachi Electronics Engineering Co., Ltd.) in a measurement voltage700V range.

[0075] The results of the measurement are shown in FIG. 6. FIG. 6 is agraph representing the relationship between temperature decreasing rateand haze. The ordinate of the graph in FIG. 6 represents the haze levelin a unit of bit.

[0076] From the results of FIG. 6, it can be seen that as thetemperature decreasing rate becomes slower, the haze level is morereduced. For example, when the temperature decreasing rate was 20°C./sec, the haze level was 50 bits or less, which is a satisfactorylevel in terms of the device characteristics. Moreover, it can be seenthat, when temperature decreasing rate is 5° C./sec, it becomesextremely low level, i.e., 25 bits.

[0077] In addition, the inventors of the present invention preciselyinvestigated the relationship between the temperature decreasing rate inthe heat treatment and temperature region for the temperature decrease.In heat treatment experiments similar to those mentioned above,temperature decrease was performed at a temperature decreasing rate of5° C./sec from the maximum temperature of 1200° C., and the temperaturedecreasing rate was accelerated to 60° C./sec when the temperaturereached to a predetermined temperature during the temperature decrease.Then, haze of wafer surfaces was measured after the heat treatment inthe same manner as described above. The results of the measurement areshown in FIG. 7.

[0078]FIG. 7 is a graph representing the relationship between thetemperature at which the temperature decreasing rate was changed to ahigher rate and haze of the wafer surfaces. As shown in FIG. 7, when thetemperature decreasing rate was changed to a higher rate in thetemperature range of 1200° C. to around 750° C., the haze level of thewafer surfaces was degraded compared with the case where the lowtemperature decreasing rate was maintained. However, it can be seenthat, when the temperature decreasing rate was changed to a higher rateafter the temperature was decreased to a temperature region below 700°C., haze of wafer surfaces was not affected at all even when thetemperature decreasing rate was accelerated thereafter.

[0079] That is, haze of wafer surfaces can sufficiently be improved byusing a temperature decreasing rate of 20° C./sec or less in the regionof from the maximum temperature in the heat treatment to 700° C., and itis not affected by the temperature decreasing rate in the temperatureregion below 700° C.

[0080] Although detail of the cause of such a phenomenon is unknown, itis thought that it is caused by such a mechanism as described below.

[0081] That is, if the heat treatment under a reducing atmospherecontaining hydrogen is performed at a high temperature of, for example,1200° C. or higher, it becomes likely that a step-like shape is formedon the surface.

[0082] These steps correspond to plane orientations that are differentfrom the initial plane orientations of wafers and generated at theatomic level. The shape of such steps formed at the maximum temperatureare maintained, if the temperature decreasing rate is fast. However, asthe temperature decreasing rate becomes slower, surface energy is morestabilized due to the effect of migration, and thus the shape isflattened. As a result, it is thought that difference of the haze levelis generated after the heat treatment depending on the temperaturedecreasing rate.

[0083] Therefore, it is considered that, if the temperature is slowlydecreased from the maximum temperature of the heat treatment to about700° C., migration is not caused at all in the temperature region blowthat level, and hence the haze level of wafers is not affected even byrapid cooling at an accelerated temperature decreasing rate.

[0084] Based on the studies explained above, it was found that COP andhaze of wafer surfaces can be improved in a method for heat treatment ofsilicon wafers under a reducing atmosphere containing hydrogen using arapid heating and rapid cooling apparatus, by the simple method, i.e.,by using a temperature decreasing rate of 20° C./sec or less for thetemperature range of from the maximum temperature in the heat treatmentto 700° C. In this method of the present invention, conditions of thehigh temperature heat treatment themselves are not changed like theaforementioned multi-step heat treatment of Japanese Patent ApplicationNo. 10-176693, and only the temperature decreasing rate, i.e., coolingintensity, is changed. Therefore, it is an extremely simple heattreatment method.

[0085] Further, since the heat treatment method of the present inventionuses a relatively slow temperature decreasing rate, there is arisenanxiety for reduction of heat treatment efficiency. However, iftemperature decrease from the maximum temperature of 1200° C. isconsidered, while it takes about 20 to about 40 seconds to decrease thetemperature to room temperature with a usual temperature decreasing rateof 30-60° C./sec, it takes only about 60 seconds or less when thetemperature decrease is attained at a rate of 20° C./sec and 4 minutesor less even when the temperature decrease is attained at a rate of 5°C./sec. Comparing with the usual heat treatment methods not using an RTAapparatus (for example, resistance heating method), which require 7-8hours for heat treatment of 1 cycle, the treatment time for each wafercan be shortened by the heat treatment according to the method of thepresent invention, while it depends on the number of wafers to betreated.

[0086] Further, if it is desired to shorten the heat treatment time,there can be used a temperature decreasing rate for the region below700° C. in the heat treatment faster than the temperature decreasingrate for the region from the maximum temperature to 700° C. For example,a wafer surface of a haze level comparable to that obtained bytemperature decrease performed with a temperature decreasing rate of 5°C./sec for the whole temperature range can be obtained by performing thetemperature decrease at a temperature decreasing rate of 5° C./sec fromthe maximum temperature of 1200° C. to 700° C., and then at a rate of60° C./sec in the region below 700° C., although such temperaturedecrease takes only 2 minutes or less. Moreover, it is also possible toperform the temperature decrease at a rate of 70° C./sec or more byturning off a power source of lamps when the temperature reaches 700° C.

[0087] As for the atmosphere for the heat treatment using the rapidheating and rapid cooling apparatus, 100% of hydrogen or a mixed gasatmosphere of hydrogen and argon and/or nitrogen can be used. By using ahigher hydrogen concentration, higher effect of improving surfaceroughness of wafer surfaces such as COP and haze can be obtained. On theother hand, by increasing the concentration of argon or nitrogen, therecan be attained an advantage of easy handling.

[0088] The present invention will be further explained with reference tothe appended drawings hereafter. However, the present invention is notlimited by this explanation.

[0089]FIG. 8 is a schematic view of an exemplary heat treatmentapparatus for performing the heat treatment according to the presentinvention. As shown in FIG. 8, a heat treatment apparatus 10 has achamber 1 in which the heat treatment is performed, and a support stand2, on which wafers W to be subjected to the heat treatment are placed,is disposed in the chamber 1.

[0090] Heaters 3 are disposed outside the chamber 1, so that theysurround the chamber 1. The chamber 1 has an gas intake 4, to which anargon feed source 7 and a hydrogen feed source 8 are connected through amixer 6, so that an atmospheric gas consisting of each of the gasesalone or a mixed gas of a desired mixing ratio can be introduced intothe chamber 1. Further, the chamber 1 has an exhaust port 5, from whichexhaustion is attained.

[0091] Now, a method for heat treatment of the wafer W using theaforementioned heat treatment apparatus 10 will be explained. The wafersW are first placed on the support stand 2, which is disposed in thechamber 1. Then, argon is fed into the chamber 1 through the gas intake4 from the argon feed source 7. In this case, the atmosphere may be amixed gas atmosphere that contains hydrogen introduced from the hydrogenfeed source 8 through the mixer 6. As mentioned above, the mixing ratioof hydrogen is preferably 30% by volume or less in order to preventetching of wafers.

[0092] After the air in the chamber 1 is fully substituted with thepredetermined atmospheric gas, electric power supplied to the heaters 3is increased to heat the chamber 1 to a desired temperature, and thistemperature is maintained for a predetermined period. The term “tomaintain a temperature” used herein just means to maintain a temperaturewithin the temperature region for the heat treatment, and it may beincreased or decreased during the heat treatment as required.

[0093] When the heat treatment of the wafers W is finished, the hydrogenconcentration of the atmosphere is increased as required in order toreduce microroughness of the surface of the wafers W. In this operation,the mixing ratio of hydrogen fed from the hydrogen feed source 8 isincreased by the mixer 6 to control the hydrogen concentration in theatmosphere to be 1-60% by volume. While a mixed gas of the desiredhydrogen concentration is supplied from the gas intake 4, the electricpower supplied to the heaters 3 is stopped or reduced to decrease thetemperature at a predetermined temperature decreasing rate. When thetemperature is decreased to a predetermined temperature, the wafers Wand the support stand 2 are taken out from the chamber 1.

[0094] By performing heat treatment of silicon wafers as describedabove, there can be obtained a silicon wafer with a thick defect-freewafer surface layer and improved microroughness and haze of the wafersurface. Furthermore, since the temperature decreasing of the heattreatment according to the method of this present invention may beperformed by using only a very small amount of hydrogen, the safety ofthe heat treatment process can also be secured.

[0095] Another embodiment of the present invention using an RTAapparatus will be explained.

[0096] Examples of the rapid heating and rapid cooling apparatus forsilicon wafers used for the present invention include apparatuses suchas lamp heaters for heat radiation. As an example of commerciallyavailable apparatuses, for example, Model SHS-2800, Steag MicrotecInternational, can be mentioned. These apparatuses are not particularlycomplicated, and are not expensive either.

[0097] Now, an example of the rapid heating and rapid cooling apparatus(RTA apparatus) for silicon single crystal wafers used in the presentinvention will be explained. FIG. 9 is a schematic view of the RTAapparatus.

[0098] The heat treatment apparatus 20 shown in FIG. 9 has a chamber 11consisting of quartz, and a wafer is heat treated within this chamber11. Heating is achieved by heat lamps 12, which are disposed under andover the chamber and at left and right of the chamber so that theyshould surround the chamber 11. Electric power supplied to these lamps12 can be independently controlled.

[0099] As for the gas supplying side, a non-illustrated hydrogen gasfeed source, argon feed source and nitrogen gas feed source areconnected, so that these gases can be mixed in an arbitrary ratio andsupplied into the chamber 11.

[0100] An auto shutter 13 is provided at the gas exhausting side, and itshuts the inside of the chamber 11 off from the outer air. The autoshutter 13 has a wafer insertion port not shown in the figure which canbe opened and closed by a gate valve. The auto shutter 13 is alsoprovided with a gas exhausting outlet, so that the atmospheric pressurein the furnace can be controlled.

[0101] The wafer W is placed on a three-point supporting part 15 formedon a quartz tray 14. A buffer 16 made of quartz is provided at the gasinlet side of the tray 14, so that it can prevent the wafer W from beingdirectly blown by the introduced gas flow.

[0102] Further, the chamber 11 is provided with a special window fortemperature measurement, which is not shown in the figure, and thetemperature of the wafer W can be measured by a pyrometer 17 installedin the outside of the chamber 11 through the special window.

[0103] By using the heat treatment apparatus 20 mentioned above, theheat treatment for rapid heating and rapid cooling of silicon wafers isperformed as follows.

[0104] First, the wafer W is inserted into the chamber 11 from theinsertion port and placed on the tray 14 by a wafer handling apparatusnot shown in the figure. Then, the auto shutter 13 is closed. The insideof the chamber 11 is filled with a reducing atmosphere containinghydrogen at a predetermined ratio.

[0105] Subsequently, electric power is supplied to the heat lamps 12 toheat the wafer W to a predetermined temperature, for example, 1100° C.to the melting point of silicon, in particular, a temperature below1300° C. In this operation, it takes, for example, about 20 seconds toattain the desired temperature. Then, the wafer W is maintained at thetemperature for a predetermined period of time, and thus the wafer W canbe subjected to a high temperature heat treatment.

[0106] When the predetermined time has passed and the high temperatureheat treatment was finished, output of the lamps 12 is reduced to lowerthe temperature of the wafer W. The method for heat treatment of thepresent invention is a method characterized in that it uses atemperature decreasing rate of 20° C./sec or less for the region of fromthe maximum temperature to 700° C. When this method is performed, it issufficient only to lower the temperature decreasing rate to 20° C./secor less, while measuring the temperature of the wafer W by a pyrometer17. This temperature decrease is conventionally performed at atemperature decreasing rate of 30-60° C./sec, taking about 20 to about40 seconds. Therefore, the method of the present invention can beperformed without substantially modifying conventionally used RTAapparatuses.

[0107] Finally, after the temperature decrease of the wafer is finished,the wafer is taken out by the wafer handling apparatus to finish theheat treatment.

[0108] If it is desired to shorten the heat treatment time, for example,the output of the lamps 12 can be turned off to rapidly cool the waferW, after the temperature of the wafer measured by the pyrometer 17 isbelow 700° C. Alternatively, by taking out the wafer and transferring itto a space at room temperature by the wafer handling apparatus, thetemperature decreasing rate in the region below 700° C. can also beaccelerated and hence the heat treatment time can be shortened.

[0109] The present invention will be specifically explained hereafterwith reference to the following examples of the present invention andcomparative examples. However, the present invention is not limited bythese.

EXAMPLE 1, COMPARATIVE EXAMPLES 1-5

[0110] A plurality of silicon wafers of the same specification wereprepared and heat treatments thereof were performed, in which each waferwas inserted into a heat treatment furnace, heated to 1200° C. at atemperature increasing rate of 10° C./min, maintained at 1200° C. for 60minutes, and then cooled at a temperature decreasing rate of 3° C./min.During the temperature increasing, retention of constant temperature andtemperature decreasing, the atmospheric conditions were changed for eachwafer to perform the heat treatments. Effect of the heat treatment wasevaluated by measuring P-V value, haze, COP (Crystal OriginatedParticle) density of wafer surface and COP density of wafer surfacelayer of a depth of 0.5 μm from the wafer surface for each wafer afterthe heat treatments.

[0111] The silicon wafers used were those obtained by slicing a siliconingot manufactured by the Czochralski method and subjecting the slicedwafers to mirror polishing in a usual manner, and having a diameter of 8inches and crystal orientation of <100>.

[0112] The P-V value was measured by an, AFM (Atomic Force Microscope,NanoScope-II, tradename of Digital Instrument Inc.) for a 2 μm squaremeasurement area. Haze (ppm) was measured by using SP-1 produced byKLA/Tencor Co., Ltd.

[0113] The measurement of COPs on a wafer surface was performed bycounting number of COPs for COPs present on the wafer surface and havinga size of 0.1 μm or more using a particle counter, LS-6030 (tradename ofHitachi Electronics Engineering Co., Ltd.) in a 700V range.

[0114] The measurement of COP density within the depth of 0.5 μm fromthe wafer surface was performed by allowing growth of a thermal oxidefilm having a thickness of about 1 μm on the wafer surface and countingCOPs through the oxide film by the aforementioned particle counter. Thatis, since increase of the measured value obtained after the oxidationcompared with that obtained before the oxidation corresponds to thetotal of COPs contained in the layer of a depth of about 0.5 μm from theoriginal silicon surface, the COP density can be obtained as describedabove.

[0115] The results of the measurement obtained as described above areshown in Table 1. TABLE 1 Atmospheric condition during Atmospherictemperature condition increase and during COP density (≧0.1 μm) constanttemperature Haze P-V value Surface Depth of 0.5 μm temperature decrease(ppm) (nm) (COP/cm²) (COP/cm³⁾ Example 1 100% Argon  1% Hydrogen 0.400.73 0.15 1.5 × 10³  99% Argon Comparative No heat No heat 0.30 0.802.20 1.5 × 10⁴ Example 1 treatment treatment Comparative 100% Hydrogen100% Hydrogen 0.31 0.76 0.14 2.0 × 10³ Example 2 Comparative 100% Argon100% Argon 1.42 1.33 0.15 1.5 × 10³ Example 3 Comparative 100% Argon100% Hydrogen 0.35 0.71 0.15 2.0 × 10³ Example 4 Comparative 100% Argon 80% Hydrogen 0.37 0.72 0.15 1.9 × 10³ Example 5  20% Argon

[0116] From the results of Table 1, it can be seen that the siliconwafer subjected to the heat treatment according to the method for heattreatment of the present invention (Example 1) has a P-V value and hazecomparable to those of the wafer subjected to heat treatment using onlythe hydrogen atmosphere (Comparative Example 2), and its wafer surfaceshows little microroughness and hence superior flatness.

[0117] Moreover, this wafer has a good surface COP density comparable tothat of the wafer subjected to heat treatment only with the argonatmosphere (Comparative Example 3), and it also shows a COP density fora depth of 0.5 μm comparable to that obtained with only the argonatmosphere. That is, it is presumed that this wafer had a thickdefect-free layer, although it was subjected to the heat treatment usingthe atmosphere containing hydrogen as the atmosphere during thetemperature decreasing, and etching of the silicon wafer surface did notsubstantially occur.

[0118] From the results mentioned above, it can be seen that the wafersubjected to the heat treatment of the present invention is a siliconwafer of higher quality of which crystal defects on the surface and inthe surface layer are eliminated without degrading microroughness on thewafer surface etc., compared with the wafer before the heat treatment(Comparative Example 1).

[0119] On the other hand, while the wafer obtained with temperatureincrease and retention of constant temperature in the argon atmosphereand temperature decrease in 100% by volume of hydrogen (ComparativeExample 4) and the wafer obtained with temperature decrease in 80% byvolume of hydrogen and 20% by volume of argon (Comparative Example 5)showed good P-V values and haze, their COP densities at a depth of 0.5μm from the wafer surfaces were degraded compared with the wafer of thepresent invention.

[0120] This is considered to be caused because the hydrogenconcentration in the atmosphere during the temperature decreasing washigh, and thus etching of wafer surfaces were generated to reduce thethickness of defect-free layers of the wafer surface layers to 0.5 μm orless.

EXAMPLE 2

[0121] A silicon wafer obtained by the Czochralski method was subjectedto a heat treatment using an RTA apparatus at 1200° C. for 10 secondsunder 100% hydrogen atmosphere. The temperature decreasing rate duringtemperature decreasing from the maximum temperature of the heattreatment was 20° C./sec according to the method of present invention.Haze on the surface of the silicon wafer was measured after the heattreatment.

[0122] For the heat treatment, the aforementioned RTA apparatus (rapidheating and rapid cooling apparatus Model SHS-2800, Steag MicrotecInternational) was used.

[0123] The silicon wafer used was one obtained by slicing a siliconingot manufactured by the Czochralski method and subjecting the slicedwafer to mirror polishing in a usual manner, and having a diameter of 6inches and crystal orientation of <100>.

[0124] Haze was measured by using a particle counter, LS-6030 (tradenameof Hitachi Electronics Engineering Co., Ltd.) in a measurement voltage700V range. The haze level was represented in a unit of bit.

[0125] As a result of the measurement, the wafer of Example 2 was foundto have a haze level of about 49 bits, which was markedly improvedcompared with one obtained with high-speed cooling, and was a levelcausing no problem in the device production. Further, the heat treatmenttime was prolonged by only 40 seconds or less compared with the casewhere temperature decreasing was performed at the rate of 60° C./sec ina conventional method. This does not greatly affect the productivity,and the productivity can be improved compared with the case using theresistance heating technique.

EXAMPLE 3

[0126] A silicon wafer was subjected to a heat treatment in the samemanner as Example 2, except that a temperature decreasing rate of 5°C./sec for the region of from the maximum temperature of 1200° C. to700° C. and a temperature decreasing rate of 60° C./sec for the regionbelow 700° C. were used. The heat treatment and the haze measurementwere performed in the same manner as Example 2 except for theaforementioned modifications.

[0127] As a result of the measurement, the wafer of Example 3 was foundto have a haze level of about 25 bits, which is markedly improved andwell meets to higher integration degree in future.

COMPARATIVE EXAMPLE 6

[0128] A silicon wafer was subjected to a heat treatment in the samemanner as Example 2, except that a temperature decreasing rate of 50°C./sec was used for the region of from the maximum temperature of 1200°C. The heat treatment and the haze measurement were performed in thesame manner as Example 2 except for the aforementioned modification.

[0129] As a result of the measurement, the wafer of this comparativeexample was found to have a large haze level of 95 bits, which isexpected to degrade the electric characteristics of wafers such as oxidedielectric breakdown voltage and carrier mobility.

[0130] The present invention is not limited to the embodiments describedabove. The above-described embodiments are mere examples, and thosehaving the substantially same structure as that described in theappended claims and providing the similar functions and advantages areincluded in the scope of the present invention.

[0131] For example, while the aforementioned embodiments are mainlyexplained for the cases where the heat treatment was performed forsilicon wafers containing many crystal defects such as COPs before theywere subjected to the heat treatment, the method for heat treatment ofthe present invention can be used for silicon wafers with few crystaldefects with a purpose of improving microroughness on the wafersurfaces.

[0132] Further, the aforementioned embodiment using an RTA apparatus wasexplained mainly for the effect of improving haze on wafer surfaces andsimultaneously reducing COPs of wafers. However, the effects of themethod for heat treatment of the present invention are not limited tothe improvement of haze, but it can also improve surface conditionsrepresented by the P-V values of wafer surfaces (maximum differencebetween peaks and valleys) or other parameters, and improve flatness ofwafer surfaces.

[0133] Further, while the embodiments mentioned above were explained forthe heat treatment of silicon wafers having a diameter of 6 inches or 8inches, the present invention can of course be used regardless of thewafer diameter, and can satisfactorily be used for a silicon waferhaving a diameter of, for example, 12 to 16 inches or more, which areexpected to be used in future.

1. A method for heat treatment of silicon wafers, wherein a siliconwafer is subjected to a heat treatment at a temperature of from 1000° C.to the melting point of silicon in an inert gas atmosphere, andtemperature decreasing in the heat treatment is performed in anatmosphere containing 1-60% by volume of hydrogen.
 2. The method forheat treatment of silicon wafers according to claim 1, wherein the inertgas atmosphere consists of an argon atmosphere or an argon atmospherecontaining 30% by volume or less of hydrogen.
 3. A method for heattreatment of silicon wafers under a reducing atmosphere containinghydrogen by using a rapid heating and rapid cooling apparatus, whereintemperature decreasing rate from the maximum temperature in the heattreatment to 700° C. is controlled to be 20° C./sec or less.
 4. Themethod for heat treatment of silicon wafers according to claim 3,wherein the temperature decreasing rate in a region below 700° C. in theheat treatment is faster than the temperature decreasing rate from themaximum temperature to 700° C.
 5. The method for heat treatment ofsilicon wafers according to claim 3 or 4, wherein the reducingatmosphere containing hydrogen is 100% hydrogen or a mixed gasatmosphere of hydrogen with argon and/or nitrogen.
 6. A silicon waferwhich is subjected to a heat treatment by the method according to anyone of claims 1-5.
 7. A silicon wafer which has a crystal defect densityof 1.0×10⁴ defects/cm³ or more in a wafer bulk portion, a crystal defectdensity of 1.0×10⁴ defects/cm³ or less in a wafer surface layer of adepth of 0.5 μm from the surface, a crystal defect density of 0.15defects/cm² or less on a wafer surface and surface roughness of 1.0 nmor less in terms of the P-V value.
 8. A method for heat treatment ofsilicon wafers, comprising: subjecting a silicon wafer to a heattreatment at a temperature of from 1000° C. to the melting point ofsilicon in an argon atmosphere; increasing the mixing ratio of hydrogenin the atmosphere to control the hydrogen concentration to be 1-60% byvolume; and decreasing the heat treatment temperature in the atmosphere.9. A silicon wafer which has a crystal defect density of 1.0×10⁴defects/cm³ or more in a wafer bulk portion, a crystal defect density of1.0×10⁴ defects/cm³ or less in a wafer surface layer of a depth of 0.5um from the surface, a crystal defect density of 0.15 defects/cm² orless on a wafer surface and surface roughness of 1.0 nm or less in termsof the P-V value.